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The greedy approach of removing a book without looking at the whole array may not always give the correct result. Consider the example array: 4, 1, 1, 9, 3 with t = 6. The best subarray here is 4,1,1 but your algorithm eliminates 4 in the first step itself.

Try passing your vector arguments to functions as references (i.e. like vector<vector<int>> &grid)